Field of the Invention
The invention relates to a dampening-unit roller, such as a metering roller, particularly, of a dampening unit of a printing machine, such as an offset printing machine, in particular, having a coating on the outer cylindrical surface thereof.
The dampening unit of a printing machine, for the most part, has a plurality of rollers serving to apply a dampening film, which is as thin and as uniform as possible, onto a printing form of a printing-form cylinder. A particularly thin and uniform distribution of the dampening medium ensures a good printing result. Moreover, it is essential that infeeding of the dampening medium take place without any diffusion of printing ink into the dampening unit. This diffusion of printing ink is especially critical for arrangements wherein the rollers of the inking unit of the printing machine are connected to the dampening-unit rollers via at least one intermediate roller.
The better the hydrophilic property of the surface of the dampening-unit roller, the thinner and more uniform the application of the dampening filml.
It has become known heretofore to use surface-ground and/or polished stainless steel as surface material for such dampening-unit rollers in order to obtain a surface which is as hydrophilic as possible. Moreover, it has become known heretofore to galvanically deposit on the outer cylindrical surface of a dampening-unit roller a chromium layer which may then be ground and/or polished.
U.S. Pat. No. 4,991,501 discloses a method in which a layer of ceramic material is applied by flame spraying onto a metal roller, the flame-sprayed ceramic layer being subjected to a pore-sealing treatment in order to improve the hydrophilic property thereof. After a predetermined service life, the pore-sealing layer requires re-hydrophilizing.
In order to improve the hydrophilic condition of the surface of the dampening-unit roller, it has also become known heretofore to add alcohol, such as isopropanol or ethanol, for example, and/or alcohol substitutes to the dampening water. For the purpose of protecting the environment and in view of future legislation and regulations, respectively, a reduction in the use of alcohol is urged. Yet, there remains a demand, nevertheless, for top-grade prints for which an extremely thin and uniform layer of dampening medium is a prerequisite.